One well known approach to the conversion of solar radiation to more directly useful forms of energy utilizes a large area collector, frequently black on its exposed surface to increase absorbtion of radiation, in contact with a circulating fluid such as water. Solar radiation falling upon the collector which is often shaped as a tube or flat panel, is absorbed therein in the form of heat, the heat being transferred to the circulating fluid.
In accordance with U.S. Pat. No. 4,088,547, a solar collector is made by growing copper dentrites electrolytically on a metal substrate, for example, a copper tube, plating the dentritic surfaces with smooth copper and then plating the smooth copper-coated dendritic surfaces with a black heat absorbing coating such as black nickel or black chrome. U.S. Pat. No. 4,148,294 describes a solar collector panel made by anodizing aluminum or aluminum alloy substrate in an aqueous electrolyte until a porous oxide layer having pores in the sub-micron range is formed and by thereafter galvanically depositing microscopic elongated metal bodies, such as nickel rods, in the pores by making the substrate an electrode in an aqueous solution containing appropriate metal ions, such as nickel ions. U.S. Pat. No. 4,005,698 describes a photon energy converter which traps incident solar energy in a geometric maze whose microstructure can be analogized to the geometric configuration of an acoustic anechoic surface. The optical photon absorber surface consists of a dense forest of aligned needles, e.g., of tungsten, of dimensions of the order of visible wavelengths with a spacing between such needles of the order of several wavelengths of visible light. Such a surface is believed to absorb with a high efficiency because of multiple reflections occurring as the incident photons penetrate the needle maze in a manner similar to that in which absorption takes place in an anechoic chamber because of multiple reflections of sound.
Due to the relatively high cost of the materials and/or fabricating techniques required to provide solar collectors of the foregoing types, their prospects for widespread use are limited.
Synthetic resins which are readily thermoformable into a variety of shapes have been widely investigated as suitable materials for the manufacture of low cost solar collectors. Some commercially available varieties are inexpensive for appropriate strengths, are acceptably resistant to ultraviolet degradation and have high absorptivity for visible light but unlike the dendridic surfaces of the solar collectors described above, generally do not possess suitably low emissivity characteristics in the infrared region (i.e., 5-10 microns). The texturizing, or etching, of polymer surfaces to modify their electrical, thermal and/or chemical properties is a known technique and has been practiced, particularly for electronic applications. U.S. Pat. Nos. 4,064,030 and 4,155,826 each describes molded fluorine resin articles whose exterior surfaces have been roughened by sputter etching in vacuum or inert gas. Such surface treatment renders the articles more adherent to materials such as adhesives, paints or inks applied thereto. U.S. Pat. No. 4,092,442 discloses a plasma-etched polyimide mask which is used in processes using either positive or negative electron beam lithography which require sensitive resists. According to U.S. Pat. No. 4,229,233, plasma etching with oxygen has been used to reduce the reflectivity of semiconductor substrates, primarily silicon, from which photovoltaic cells are fabricated. Plasma-etched thin film protective coatings, e.g., of polyimide, for sensitive electronic devices such as integrated circuits are known (viz., DuPont descriptive literature for the Pyraline polyimide coatings). Heretofore, the texturizing of a thin film resin coating to improve the absorption and emission characteristics of a solar collector has not been described.